Document NO2L8Roz2qzkx3b5kbyGLa3g

t * I t i 492 CHAPTER 33 1960 Guide are not always known, another convenient method of ex pressing the relation of Equation 5 is as follows: 100(0, - CQ/2) Percent excess air N, X 0.264 - (Oi -- CO/2) (7) As measurement standards for gaseous fuels are almost universally expressed in cubic feet, Equation 8 may be em ployed for computing excess air on a percentage baas for Percent excess air (V - CO*) x ioop- CO, (8) where V = ultimate carbon dioxide, percent of flue gases result ing from perfect combustion. CO, " carbon dioxide content of flue gases, percent. P dry products from perfect combustion, cubic feet per cubic foot of gas burned. A = air theoretically required for complete combustion, cubic feet per cubic foot of gas burned. As the ratio of P/A is approximately 0.9 for most city gases, a value of 90 may be substituted for 1Q0(P/A) in Equation 8 for rough calculation. Carbon-hydrogen ratios of different fuels vary consider ably, hence the maximum or ultimate CO, attainable a1w> varies. Where they are unknown, theoretical maximum CX), values may be calculated from a flue-gas analysis by use of Equation 9. Maximum theoretical % CO, % CO, in flue gas sample X 100 ^ . /O, io same sample\ \ 0.21 ) Approximate maximum CO, values for perfect combustion of several common typ& of fuel are shown in Table 9 to gether with values of CO, that will be attained with different Table 9 .... Approximate Maximum Theoretical COt Values, and CO, Values for Various Fuels with Different Percentages of Excess Air type of Awl Maximum Percent CO, of Given Theoretical or Excest Air Value, Utimote Percent CO, 20% 40% 60% Coke Anthracite Bituminous Coal No. 1 and 2 Fuel Oil No. 6 Fuel Oil Natural Gas Carburetted Water Gas Coke Oven Gas Mixed Gas (Natural and Carburetted Water Gas) Propane Gas (Commercial) Butane Gas (Commercial) 21.0 20.2 18.2 15.0 16.5 12.1 17.2 11.2 15.3 13.9 14.1 17.5 16.8 15.1 12.3 15.0 14.4 12.9 10.5 13.0 12.6 11.3 9.1 13.6 9.9 14.2 9.2 11.6 8.4 12.1 7.8 10.1 7.3 10.6 6.8 12.6 10.5 9.1 11.4 9.6 8.4 11.6 9.8 8.5 amounts of excess air. Desirable values to be attained in practice depend upon the fuel, the method of firing, and other considerations. In general, fuels burned in suspension, such as gas, oil. and pulverized coal, can be burned with a lower amount of excess air than fuels burned on grates. To produce heat efficiently by burning any common fuel, a number of basic requirements must be met: (1) adequate heat absorbing surface of proper shape and construction is necessary in the appliance, (2) heat transfer surfaces must be clean, (3) a minimum amount of excess air must be present, (4) air employed for combustion and combustible gases must be property mixed, and (5) flue-gas losses must be reduced to a safe minimum. If insufficient heating surface is employed, or if heat trans fer surfaces are covered with soot, ash, or scale, flue losses will generally be excessive due to the large amount of sensible heat escaping to the chimney. Too much excess air dilutes flue gases excessively and increases sensible flue gas loss. On the other hand, a deficiency of excess air will in all prob ability cause incomplete combustion, and consequent escape of some unburned combustible gases from the appliance. Highest combustion - efficiency is obtained when sufficient excess air is supplied and properly mixed with the com bustible gases. Even with appliances connected to an effec tive flue every reasonable precaution should be taken to in sure as complete combustion as possible at all times. This is of vital importance in unvented equipment such as a gas space heater, for example It is considered good practice to supply from 20 to 50 percent excess air, the exact amount depending on the kind of fuel burned and especially on the type of equipment in winch it is utilized. Flue Gas Analysis As radiation, convection, and conduction losses from com mon types of central heating appliances are largely em ployed in heating occupied spaces, flue-gas losses logically become the item of principal concern. Another reason for their importance is the fact that they are usually larger than all other heat losses combined. These important considera tions and others indicate not only the desirability but ab solute necessity of a reasonably accurate method for deter mining flue losses even if only approximate, operating efficiencies arc to be computed. Customary procedure in arriving at flue-gas losses is to make an analysis of a representative sample of flue gases and to measure their temperature. This information will enable the observer to compute the amount of flue gases produced, the excess air, the actual quantity of air supplied for com bustion, and the flue losses. While the analysis of flue gases has been well described in several governmental and other scientific publications, the subject is of such importance that it warrants brief repetition here. Carbon dioxide and oxygen content are of principal interest in determining flue .losses. Either or both of these values may be employed in such cal culations. While the former constituent is probably most commonly employed, special considerations may make the latter of greater interest. Fortunately, both can be deter mined readily by use of an Ors&t gas analysis apparatus, a device of reasonably simple construction and design. For held testing and burner adjustment, simple portable devices are available for determining carbon dioxide only. See Chap ter 44 for details regarding the operation of the Orsat ap paratus. The weight of dry flue gas per pound of fuel burned is Fuels and Combustion widely used in combustion loss calculations. For solid fuels this item may be determined by application of Equation 10. Pounds dry flue gas per pound fuel 11CQ, + SOi + 7(CO + N) 3(CO + CO) (1Q) Values for CO., O*, CO, and N* are percentages by volume from the flue gas analysis, and C is the weight of carbon burned per pound of fuel, corrected for carbon in the ash. Total dry gas volumes of flue gases resulting from the combustion of one cubic foot of gaseous fuels for various percentages of CO. may be determined by application of Equation 11: Cubic feet dry flue gases per cubic foot fuel gas Cubic feet CO* produced ______ per cubic foot of gas burned X 100 (11) Percent CO* by analysis After obtaining the quantity of flue gases from Equations 10 or 11, the excess air quantity may be determined by sub tracting the quantity of dry flue gases which would result from perfect combustion. Computations of flue losses are de scribed in the next section on Heat Balance. Application of the preceding equations and tables are il lustrated by Examples 1 and S. Example 1: The analysis of the flue gases resulting from the burning of a natural gas is 10.0 percent CO., 3.1 percent O*, and 889 percent N. by volume. The analysis of the fuel is 90 percent CH, 5 percent N*, and 5 percent' TVH. by volume. Find U the maximum theoretical percent .CO. and the percent excess air. Solution: From Equation 9: (I0,0)(100) U m~(k) 118% CO, From Equation 8, (118 - 10.0) X 90 Percent Excess Air = 16.2 10 Example t: For the analyses in Example 1 find, per cubic foot of fuel gas, the cubic feet of dry air required for com bustion, the cubic feet of each constituent in the flue ga>, and the total volume of dry and wet flue gases. Solution: From Equation 3 (or Table 7) the volume of dry air required for combustion is: (983) (CEL) + (1658)(C*H.) = 953 X 0.90 + 1658 X 055 - 9.41 cu ft/cu ft gas. From Table 7, the constituents per cubic foot of gas are: Nitrogen, N* : From methane = (0.9 CH.X953 - 2.0) = 6.78 From ethane - (055 OH.X 16.68 - 35) = 056 Nitrogen in fuel = 055 Nitrogen in excess air = 0.791 X 0.162 X 9.41 = 120 Total Nitrogen 859 cu ft Oxygen, O,: Oxygen in excess air = 0.209 X 0.162 X 9.41 -- 052 cu ft Carbon dioxide, CO* : From methane = (09 CH*)(15) -- 090 From ethane = (055 CH*)(45/25).= 0J0 Total Carbon Dioxide 150 cu ft 493 Water vapor, HjO (does not appear in Orsat analysis): (OS CHi)(25) = 18 (055 C*H)(6.0/2.0) = 0.15 Total water vapor = 185 cu It Total volume of dry gas per cubic foot of gas: 8.69 + 052 + 150 = 10J>1 cu ft Total volume of wet gases per cubic foot of gas (neglecting water vapor in combustion air): 10.01 + 195 - 1196 cu ft The cubic feet of dry flue gas per cubic foot of fuel gas may also be computed from Equation 11 as follows: (1.00X100) fAA ,, HEAT BALANCE The usual practice in analysing the performance of heat ing appliances is to make an accounting, insofar as possible, of the disposition of all heat units available in the quantity of fuel burned. This accounting is called a heat balance. Various components of this balance are generally expressed in terms of Btu per pound of fuel burned, or as a percentage of its calorific value. Components of special interest are listed as items 1 to 7 inclusive. 1. Useful heat transferred to beating medium, usually com puted by determining the rate of flow of the heating fluid through the heating device, and the change in enthalpy of the fluid (heat added) between the inlet and outlet. 2. Heat loss in the dry chimney gases. hi - tec* (t, - t.) (12) 3. Heat loss in water vapor formed by the combustion of hydrogen. hi = ~ (1089 - U + 0.4551,) (13) 4. Heat loss in water vapor in the air supplied for com bustion. hi = 0.455 if w. (1, - 1.) (14) 5. Heat loss from incomplete combustion. ** " 10W3C (coT+lio) (I5) 6. Heat loss from unburned carbon in the ash or refuse. * -U000, (fe - c) H6) * A value of 14600 applies in calculating ash pit loss; in ealeu- ' fating heat of formation of carbon compounds use 14093 Btu per lb. 7. Radiation and all other unaccounted for losses. Radiation and convection losses from a heating appliance are not usually determined by direct measurement. For this reason they, together with any other loses not measured, are determined by subtracting the total of items l to 6 from the heat of combustion of the fuel. -If the heating appliance is located within the heated space, however, - radiation and convection losses may be considered as useful heat rather